Shedding Light on UFAD

Modern corporations restructure and reorganize at an increasing rate, and their offices must keep pace.

Furthermore, such office areas demand more—and cleaner—power than ever before. Under these circumstances, many engineers are faced with the challenge of designing a power system that meets the ever changing electrical demands of technology, while still being flexible enough to accommodate corporations with churn rates as high as 30%.

As explored in CSE's last issue, one solution is to use an open office plan with an underfloor air-distribution system. But how, specifically, does UFAD affect the design of lighting, power, data and phone lines?

First, raised-floor installations allow for the possibility of a flush-mounted box in the floor of every cubicle, which can be easily relocated every time the office layout changes.

Each floor box should minimally be a five-slot box, utilizing at least two slots for power and the other three slots for voice, data and video (VDV) ports. Each power slot should include a duplex receptacle, with each outlet on a separate circuit. Flexibility for power connections can be provided by utilizing off-the-shelf components with prewired or preconfigured cables, connectors and underfloor power-distribution nodes. Power whips can plug into the floor box and the distribution node. This plug-and-play feature makes box relocation as simple as picking up the box—with its floor tile—unplugging it, moving it and plugging it back in. To accommodate this setup, whips and VDV cabling should be specified with up to 10 ft. of slack coiled at the floor box to provide for the relocation of each box within a 10-ft. radius. Power distribution nodes can then be hardwired through metallic conduit back to the respective panels for each area.

When a UFAD system is used, all underfloor boxes must be rated for "environmental air" per the National Electrical Code (NEC). All power conductors should similarly be contained in prewired, flexible metallic conduit/armored cable. The typical electrical load for a 10-ft. by 10-ft., VDV-connected office cubicle could be as high as 600 to 700 watts including: personal computers, computer monitors, task lights, auxiliaries such as a calculator or pencil sharpener and an ISDN/IP local-powered phone. Therefore, two 20-amp, 120-volt, AC single-phase circuits can feed up to five standard cubicles with one five-slot floor box each.

One of the duplex receptacle circuits should be considered as an isolated ground (IG) or as conditioned power dedicated for personal computer equipment. The other duplex receptacle circuit should be used for noncomputer equipment such as lights, calculators and other auxiliaries. The IG or conditioned power circuit can be color coded or labeled for computer use only. These recommendations, however, do not include high-density power areas of computer clusters or networked printer clusters, which require separate circuiting based upon their specific loads.

Additionally, the computer power and general-purpose power circuits should have separate paths back to the main distribution point. Any computer power risers should be sourced using K-factor-rated step-down transformers that compensate for the heating effects of power system harmonics, with a K-factor of 13 or better recommended for high-density VDV office areas. These transformers can also be specified with shielding as an added measure of protection. Consideration should be given to sizing all neutrals at 200% on the computer power risers and feeders to compensate for the additional harmonic currents generated by VDV and computer equipment.

Regardless of harmonics, all power systems should be specified with transient-voltage surge-suppression (TVSS) devices to protect against externally and internally generated transients. Suitable service entrance and distribution panel protection should be provided and consideration should be given to protection at the receptacle per application and owner requirements. TVSS devices that protect electronic components, such as personal computers, should be specified to clamp or clip the surge voltage to below 400 volts.

Grounding is also critically important in this environment, as the metallic raised floor needs to be bonded per the manufacturer's requirements to the building grounding system. One strategy is to bond the raised-floor stringers to bare copper grounding conductors at the spacing intervals recommended by the manufacturer.

With power considerations out of the way, network and communications equipment can be connected. A typical individual office will have one phone and PC; in addition, there may be a networked printer, a modem, a small TV monitor and a video-conferencing connection. Most digital or analog phones are supported with one or two pair: IP phones will require two pair and ISDN phones will require three pair. Flexibility can be provided in two ways: a zone cabling approach or a slack loop approach.

A "zone" cabling approach works well where a fixed number of Category-5e cables are hardwired to various zone termination areas of the underfloor. From these areas, Category-5e cables are then run to the floor boxes and terminated. Any changes or future cable additions are made back only to the closest zone termination area. This reduces the cost of home-running new cable back to the horizontal cross-connect (HC) closet.

An alternative is to use a "slack loop" approach, where a 10-ft. slack loop is placed at each floor box allowing the box to be moved within a 10-ft. radius without having to add cable. A benefit of this method is that it does not introduce another connection in the horizontal channel. The cable installed under the raised floor should be plenum-rated type CMP, and if cost permits, should be supported off the floor in wire-rod-type cable tray, as this helps define cable pathways and provides for easier moves or changes in the future.

Telecommunications cabling designs for high-density VDV office areas need to address bandwidth, flexibility and preparation for future applications. The typical five-slot floor box for raised-floor applications should be outfitted with a minimum of three four-pair Category-5e cables, with consideration given to an additional RG-6 coaxial drop and a one-pair fiber-optic multi-mode drop. All cables should be home-runned to the nearest telecommunications HC floor closet.

It is important to note that the Category-5e cable requirement is a minimum; consideration should be given to higher bandwidth Category-6 systems. But three Category-5e cables can support most data or voice applications, and can be cross-connected, as the application requires, in the HC floor closet. Category-5e cable will support up to 1,000 Base-T data applications including: digital or analog phones, ISDN phones, analog modems, IP phones and ISDN based video conferencing. The fiber, or coaxial drop, can support broadband video applications or very high bandwidth data applications. The voice, data and video ports, plus the additional high bandwidth port(s) will require two slots of the floor box and may require the plates to be specially punched, based upon the physical requirements of the jacks and the slot plates.

Quality lighting is another key in accommodating the design challenges of the modern, high-churn office.

The utilization of a task-ambient lighting system is one method of meeting the stringent lighting requirements set for these open-office spaces. Task-ambient schemes are so called because they typically only generate relatively low overall light levels—about 30 foot-candles—but are supplemented with specific task lighting as required. Ambient light produces a comfortable environment for working on computers or moving within the office, while the task lighting provides the higher light level required for specific jobs or individual needs.

Significant energy savings are not the only benefits achieved by not lighting the entire space at a high level. Task-ambient lighting is also well suited to environments with high churn rates. For example, linear, indirect pendants working off an 8-ft. modular design can be connected to create fixtures of various lengths that are easy to separate and rearrange. Aligning the pendants with the building structure—instead of furniture layout—allows the furniture plan to change frequently without affecting the lighting.

An additional benefit of an indirect scheme is that the light reflects off the ceiling, producing even, shadow-free illumination and limiting glare on computer screens. Calculations must be done to ensure that the ceiling uniformity ratio and the task plan-to-ceiling ratio complies with the Illuminating Engineering Society (IES) recommendations found in the IES handbook and its recommended practices 1 (RP-1).

In addition to indirect illumination schemes, those lighting VDV-intensive offices should also be aware of a relatively new lighting technology well suited to such environments. High Output T-5 lamps, or "HOT-5" lamps, for example, have dramatically impacted architectural lighting in the past few years.

HOT-5 lamps are 40% smaller than a standard 32-watt T-8 fluorescent lamp, yet produce nearly twice the amount of light. As a result, in many indirect ambient lighting systems, a single 55-watt HOT-5 lamp can replace two 32-watt T-8 lamps.

This switch, of course, allows an energy savings of 9 watts per T-8 eliminated, but the 50% reduction in the number of lamps purchased, stored, changed and disposed is the most significant factor in the long term.

The lamp's size also allows for smaller, more efficient fixtures that can be more aesthetically pleasing. HOT-5 lamps also have the same life expectancy as standard T-8 lamps, along with improved color-rendering capabilities.

But as with all new technologies, there are complications. The design temperature for HOT-5 lamps is about 10°F higher (about 85°F) than that of other fluorescent lamps. In fact, Phillips Lighting notes that its T5 HO lamps produce 5,000 lumens at 35°C (95°F) and 4,450 lumens at 25°C (77°F)—the standard test temperature for lamps. As a result, a HOT-5 lamp can lose as much as 40% of its rated light output in low temperatures—for example, if an HVAC diffuser blows 55°F air across the lamp. Therefore, the technology might not make much sense with a standard HVAC system, as lamp lumens would have to be de-rated by at least 10% because the ambient air temperature would be too low. This is not a problem, however, if UFAD is employed. Typically such systems produce an outlet air temperature of 65°F at the floor and because the system allows air to stratify, the air near the ceiling and lamps is warm and uniform in temperature, allowing the lamps to operate at peak efficiency. It should be noted that 35°C is the optimum operating temperature of most fluorescent lamps, so even standard T8s would benefit from the warmer ambient ceiling temperatures of a UFAD system.

In summary, office buildings with power densities of 1.5 watts per sq. ft. are a thing of the past. Distracting overhead lighting reflections on computer screens should also be a thing of the past. Modern offices require flexible power and lighting systems that respond to the increasing use of computers as well as high churn rates and energy/environmental concerns.

UFAD systems easily accommodate changes to cabling and wiring. Indirect lighting, combined with task-ambient lighting, eliminates glare and simultaneously reduces waste and hazardous disposal costs (mercury) by 50%. Additionally, UFAD systems allow new technology, like HOT-5 lamps, to thrive, generating real maintenance and energy savings—below 1 watt per sq. ft. But more importantly, these technologies often create more comfortable environments, ultimately improving employee satisfaction.